1. Field of the Invention
This invention relates to a weight measuring device, such as an electronic balance or an electronic scale, and more particularly to an electronic balance or an electronic scale capable of constantly measuring and displaying a weight value of a sample placed on a measuring pan, thereby allowing a user to add additional amounts of the sample little by little to the measuring pan until a desired weight value is displayed.
Incidentally, the electronic balance or electronic scale referred to herein includes a general weight measuring device for measuring a sample by detecting the weight or load of the sample as an electric signal. For example, it includes an electronic balance provided with an electromagnetic balancing mechanism, and an electronic scale using a load cell, tuning fork, electrostatic capacity and the like.
2. Description of the Related Art
When a sample is placed on a measuring pan of an electronic balance or electronic scale (hereinafter representatively referred to as an electronic balance), the measuring pan oscillates. A measured value displayed on an indicator can be read after the oscillation of the measuring pan gradually attenuates and the measuring pan becomes stable.
In reading the displayed value of the electronic balance, if the measuring pan continues to oscillate under the influence of the surrounding environment and the like, the measured value displayed on the indicator changes, thus making it difficult to read the measured value. For this reason, in order to stabilize the displayed measured value and to facilitate the reading of the measured value, some electronic balances utilize a moving average processing for stabilizing the indication by averaging the current measured data just obtained and a prescribed number of past measured data obtained during a preceding predetermined period (refer to JP-A-11-311566).
When the moving average processing is adopted in order to stabilize the indication of the measured value, it is true that the stability of the indication is improved, while the follow-up capability and response to a load change are deteriorated. When an abrupt load change occurs with the measuring pan, and if the moving average processing is active, the moving average processing operates to suppress an actual load change as well as an oscillating component that is noise. This retards the response to the load change.
In order to avoid such an inconvenience, in executing the moving average processing, a threshold value of a load change range serving as a criterion is set in advance. Then the determination of switching release and start of the moving average processing is performed on the basis of comparison result between the load change actually occurred and the threshold value of the load change range which is set in advance. That is, when the load applied to the measuring pan is changed, during the period in which a load change range is larger than the preset threshold value generated, the moving average processing is released. Whereas when the load change range becomes smaller than this threshold, the moving average processing is started.
On the other hand, in the electronic balance using a “PID control system” in which displacement detection data of a balance beam is PID-operated for feedback control, a plurality of sets of P, I and D values are previously stored so that these setting parameters can be changed optionally at any time by software. Then the measuring environment in which the balance is placed and how the balance is being used are estimated from the time-series information of the displacement detection data of the balance beam. According to the estimated result, an optimum set of the PID values is selectively set from the sets of the PID values previously stored (refer to JP-A-10-19642).
For example, although the servomechanism for balancing the balance is active, when the time-series change of the data is a monotonous increase, this is a state that sample is gradually added to the measuring pan. Thus, it is estimated that weighing operation is now in progress.
Therefore, in this case, a combination of the PID values regarding the high-speed response more important than the stability is selected so that the PID control suitable for weighing operation is executed. Further, if the time-series change is not the monotonous increase, a standard deviation of the time-series data is calculated, and a combination of the PID values which has a larger I value of the PID values for a larger standard deviation and a larger I value and a smaller P value for a shorter oscillation period is selected to execute the PID control. Further, if the oscillation period is the longest, a combination of the PID values with the larger D value is selected to execute the PID control. As described above, according to the time-series information of the displacement detection data, an optimum combination of the PID values is selected.
As described above, in the electronic balance of the related art which adopts the moving average processing as disclosed in JP-A-11-311566, whether the moving average processing should be released or executed is determined by comparison between a preset fixed load change range threshold and an actual load change range. Therefore, in the weighing operation of measuring a very small quantity of load, the moving average processing is maintained as long as the change occurred by one additional load does not reach the fixed threshold. Thus the response to the change due to the very small quantity of additional load is retarded.
Usually, in the weighing operation, as the load approaches a target measured value, additional load is decreased each time. Therefore, if the threshold value is set at a relatively large value, the moving average processing becomes active in an earlier stage. As a result, addition of a very small quantity of load is repeated many times with a retarded response. This takes a long time for measurement, deteriorates the working efficiency, and also is likely to result in excessive measurement or insufficient measurement.
On the other hand, if the threshold value of the fixed load change range is set at a sufficiently small value, in the weighing operation, the load change does not exceed the threshold value until the additional load in each additional operation becomes relatively small. Thus, the load can be measured with good response, while just with a small oscillation, the moving average processing is easily released, thus making the indication unstable.
Further, like the electronic balance using the PID control system disclosed in JP-A-10-19642, high-speed response and stability of indication can be optimized to some degree according to the surrounding environment by the method of selecting the optimum PID values of the PID control according to a use environment.
In this method, however, sets of suitable PID values need to be prepared according to the use environment by actual measurement and the like. This operation requires knowledge about the PID values and labor and time.
Further, the electronic balance sometimes generates a creeping phenomenon that even after the oscillation becomes calm after the load change, a slow change continues for a while as shown in FIG. 3. The creeping phenomenon is sometimes attributable to, for example, deformation of a “Roverbal” mechanism used in a load detecting section, or deformation of a load cell itself or the attached member (bonding layer and the like) for fixing the load cell in an electronic balance using the load cell system.
In the electronic balance disclosed in JP-A-10-19642, when the servomechanism for balancing the balance is active, and if the time-series change of data is a monotonous increase, it is determined that the weighing operation is being executed without exception. Therefore, even if the monotonous increase of the time-series change of data is due to the creeping phenomenon, it is determined that the weighing operation is being executed, thus causing the inappropriate control.
Specifically, since the electronic balance is controlled without distinguishing the cause of data change, in the weighing operation requiring a high-speed response, the stabilization of the indication by moving average processing may be performed. Inversely, in the weighing operation requiring the stabilization, moving average processing may not be performed. Thus, the above electronic balance does not make an appropriate response depending on the situation and the usability is insufficient.